Hi, unfortunately this can happen sometimes. CI data is hard to find, and different add-ons may end up with slightly different values (I honestly don’t know whether the error is with FF or SimBrief, it’s possible both are correct but simply using data for different variants).
A tuning option probably won’t do much good unfortunately. CI numbers are complex; they vary with weight, altitude, headwind/tailwind, etc. In your specific flight conditions, at your planned altitude and weight, the FF may have been Mach 0.02 slower than SimBrief. But at a different weight or altitude, on a different day, it could very well be Mach 0.02 higher instead. The CI formula is way too non-linear for a simple flat correction to be of any use unfortunately.
I don’t have an easy solution for you at this time, this is just one of the limitations of having many different add-ons all based on varying amounts of incomplete data.